Interaction between members of the death receptor family and their cognate ligands induces apoptosis controlling the homeostasis of cell populations in tissues, particularly in the immune system. Although many tumor cell types are sensitive to death ligands, activation of Fas signaling also induces massive apoptosis in the liver leading to organ failure and death precluding its use for systemic anticancer therapy. Fas ligand is a 40 kDa physiological agonist of Fas signaling expressed on activated lymphocytes and many tumor cells which can also be secreted through metalloproteinase-mediated cleavage and kill the sensitive cells in autocrine and paracrine manner. Fas is a transmembrane receptor expressed on activated lymphocytes, variety of tissues and tumor cells. Fas signaling plays crucial role in regulation of the immune system by triggering autocrine suicide or paracrine death (apoptosis), suppressing immune reaction by eliminating activated lymphocytes. Upon binding, it induces p53 independent cell death through extrinsic pathway of apoptosis engaging DISC formation, caspase-8 and 10, and intrinsic (mitochondrial) apoptosis activating caspase-8 and Bid cleavage, and cytochrome release. Both apoptotic pathways lead to activation of caspase-3 and 7. Mitochondrial apoptosis is regulated by pro- and anti-apoptotic Bcl2 family members. In tumor cells, Fas signaling is often found deregulated either by absence of Fas receptor, or by constitutive activation of NF-kB resulting in the expression of anti-apoptotic genes, such as c-Flip, Bcl-2, Bcl-xL. C-Flip, an NF-kB responsive gene, has been demonstrated to inhibit caspase-8 and Fas mediated apoptosis in tumors (REFs, Kataoka et al 2000).
Upon discovery of p53 independent apoptotic mechanism through Fas, TRAIL and TNFα death receptor signaling, they seemed to be promising targets for anti-cancer therapy since tumor cells usually have impaired p53 function. A severe hepatotoxicity, however, is induced by death receptor ligands. This has hampered development of these anti-cancer therapies. While Fas agonists cause liver damage and TNF-a induces strong inflammation in liver, lungs and other organs, TRAIL is the least toxic in humans. TRAIL has therefore received more attention than other agonists for the clinical application for an anticancer treatment. Many tumors, however, are not sensitive to TRAIL therapy. Several approaches to resolve death receptor toxicity issue are currently undertaken, most of which are aimed to increase tumor sensitivity by blockage of NF-kB activity and increasing receptor expression thus reducing the amount of drug necessary for the effective therapy. Another direction is to localize the drug delivery to the tumors to minimize toxic effects on distant organs. To date, there is no reliable approach to the prevention of toxicity (including liver injury) that would allow the systemic application of death receptor agonists in clinical trials. Accordingly, there is a need in the art for methods of preventing the undesirable effects of death receptors when they are used to treat cancer. In particular, there is a need to protect the liver from these undesirable effects. There is also a need for protecting the liver from liver toxicities in general.
TLRs are found to be expressed on both epithelial and endothelial cells as well as immunocytes. At present, thirteen TLRs have been identified in mammals. Upon receptor stimulation, several common signaling pathways get activated such as NF-kB, AP-1, PI3K/AKT and mitogen-activated protein kinases (MAPK) leading to increased survival, stimulation of cell proliferation and the secretion of many cytokines with chemotactic and pro-inflammatory functions. Induction of TLR in cancer cells can be used to treat cancer, however, the distribution of different TLRs varies significantly among the various organs and cell types. This affects the cytokine profile and extent of the inflammatory response of cells. Accordingly, there is a need in the art for cancer immunotherapeutic methods that do not depend on the presence of TLR5 expression.